The invention relates to a magnetic sensor which uses the magneto-resistance effect of two magneto-statically coupled flat layers of electrically conductive ferromagnetic material having an easy axis of magnetization in a major plane of the layer, which layers are situated a small distance apart parallel with respect to each other and give a response in the form of an electric resistance variation on application of a magnetic field having a component in the planes of the layers. A non-limitative enumeration of such materials, comprising in particular ferromagnetic nickel alloys, can be found in the article "The Anisotropy in The Magnetoresistance of Some Nickel Alloys" which is published in the periodical Physica, Vol. XXV, 1959, pages 702-720.
Sensors which are formed from a layer of these materials and use this property are essentially sensitive to the value of a magnetic flux, independent of whether said flux varies or not. They may be used in magnetic field meters, magnetic rulers and as magnetic reading heads for reading magnetic record carriers. In the latter case, they enable the reading of record carriers which are moved relative to the reading head at an arbitrary relative displacement velocity, zero velocity included. They have in addition a very extensive frequency response, from zero to a few tens of megahertz, as well as an amplitude of the reading signal which is considerably larger than that which can be obtained by inductive sensors which are sensitive only to variations of the magnetic flux.
However, ferromagnetic layers having a magneto-resistance effect suffer from the disadvantage of having a low sensitivity to small magnetic fields. U.S. Pat. No. 3,860,695 describes how this disadvantage can be mitigated by using a sensor having two flat magneto-resistive layers arranged parallel to each other, which layers are connected in series with a voltage source which has a constant voltage, while the voltages set up at their ends are supplied to a difference amplifier. The layers have equal electric and magnetic properties and thicknesses, an insulation layer of at least 30 nm being present between the layers and insulating the layers electrically from each other and producing a certain magnetostatic coupling between the layers, while the layers convey measuring currents flowing in opposite directions, the measuring current through each layer energizing the relevant layer and biasing magnetically the other layer by the magnetic field induced therein. Biasing is to be understood to mean herein that the direction of magnetization in a layer in the absence of an external magnetic field is biased so as to make an angle of approximately 45.degree. with the direction of passage of the electric current, so that the operating point is moved towards the area having the largest inclination of the electric resistance-external field function. This means that, with a recording medium having a small magnetic signal an as large as possible resistance variation occurs. The direction of passage of the currents is always parallel to the largest dimension of each layer.
A disadvantage of this known configuration is that the measuring current must have a fixed value which is such that the magnetization directions induced in the layer indeed make an angle of approximately 45.degree. with the current. Moreover, this involves, in practice, that the measuring current in each magnetic sensor has to be adjusted separately due to the spreading in properties of the layers.
A further disadvantage of this known configuration is that, because of the requirement that the two series-arranged magneto-resistive layers must be fully insulated from each other electrically, their separation must not be less than approximately 30 nm, because with thinner layers the possibility of undesired interconnections occurring is considerable. At this distance, the magnetostatic coupling between the layers is not optimum and a comparatively large measuring current is necessary to realize the rotation of the magnetization which is necessary for the desired biasing.
Another disadvantage is that two ends of the magneto-resistive layers of the known sensor have to be connected separately to a difference amplifier, which further complicates the manufacture of the magnetic reading head in thin film technology.